The present invention relates to a circuit arrangement for controlling an analog voltage signal.
In halogen lamps, for example, exact maintenance of the specified lamp voltage is a fundamental precondition for long life. An increase of 5-10% above the specified lamp voltage reduces the life of halogen lamps by about 50%.
Transformers which are known from the prior art for halogen lamps have feedback between the lamp load and the frequency. If, for example, the lamp load is high, the operating frequency decreases, while, when the lamp load is low, the operating frequency rises. The stray inductance of the transformer results in a frequency-dependent voltage drop, so that the output voltage which is then provided to the lamp remains virtually constant.
The present invention is based on the object of providing a circuit arrangement which allows an analog voltage signal to be controlled in a simple manner.
According to the invention., this object is achieved by a circuit arrangement for controlling an analog voltage signal which comprises an input connection for applying the analog voltage signal, furthermore a digitizer unit which uses the analog voltage signal to produce a digital signal which has a predetermined duty ratio when the analog voltage signal is at a nominal value, a PI regulator to which, as the input signal, a nominal signal and an actual signal which is correlated with the digital signal can be supplied and at whose output a control signal for controlling the analog voltage signal can be produced, in such a manner that the control signal allows the analog voltage signal to be increased if the duty ratio is less than the predetermined duty ratio, allows the analog voltage signal to remain unchanged if the duty ratio corresponds to the predetermined duty ratio, and allows the analog voltage signal to be reduced if the duty ratio is greater than the predetermined duty ratio.
The present invention is based on the knowledge that the lamp voltage can be kept constant even for different mains voltages and lamp loads if the present lamp voltage is detected and is used to derive a suitable control signal, which can then be used for appropriate control of the frequency, the phase, the duty cycle or any other suitable variable, in order to maintain the output voltage at the nominal value. The preferred field of application is, in particular, externally controlled half-bridge circuits.
The solution according to the invention is distinguished in that it can be produced very cost-effectively and allows the lamp voltage to be detected and regulated in a manner which is virtually independent of component tolerances.
In the situation where the analog voltage signal is not yet in rectified form, an apparatus for rectifying the analog voltage signal can be provided between the input connection and the digitizer unit.
The PI regulator has a time constant Tp, with the digital signal being a signal, in particular a square-wave signal, at a frequency fd greater than 1/Tp, in particular a multiple of it. Such design ensures that the control process does not take place hectically, that is to say even taking into account irrelevant high-frequency interference, but smoothly and moderately.
The digitizer unit preferably comprises a voltage divider by means of which a partial input voltage signal can be produced which corresponds to the analog input voltage signal, a series circuit having a zener diode and a resistor, to which the partial input voltage signal can be supplied, and a switching element to which the signal at the junction point between the zener diode and the resistor can be supplied as a control signal. Such an embodiment of a digitizer unit allows the predetermined duty ratio to be varied in a particularly simple manner, to be precise by the use of different values in the voltage divider. This means that the circuit arrangement according to the invention can be used universally for controlling a large number of different analog voltage signals.
In one particularly preferred development, an optocoupler is arranged between the digitizer unit and the PI regulator. The optocoupler ensures DC isolation between the input and output of the circuit arrangement and allows the circuit arrangement according to the invention to be used even in circuits which have to comply with SELV (Secure Extra Low Voltage) conditions. In this case, the optocoupler is preferably connected in series with an output connection of the switching element. If, for example, the switching element is a transistor, the transistor current flows through the light-emitting unit of the optocoupler, and the light emitted is correlated with the digital signal.
Furthermore, the output signal from the optocoupler, for example the voltage supplied by a phototransistor arranged in the optocoupler, is preferably used to produce the actual signal for the PI regulator.
The duty ratio which is intended to be controlled is preferably varied by varying the nominal signal supplied to the PI regulator. The duty ratio may be, for example, 50:50.
In one preferred exemplary embodiment, the PI regulator is an inverting PI regulator, that is to say the nominal signal is connected to the positive input of the operational amplifier in the PI regulator, while the actual signal is connected to the negative input of the operational amplifier.
Further advantageous embodiments are described in the dependent claims.